Remove redundant code in finetune.py

Fix https://github.com/QwenLM/Qwen/issues/660

finetune.py

@@ -270,12 +270,6 @@ def train():
     if getattr(training_args, 'deepspeed', None) and int(os.environ.get("WORLD_SIZE", 1))==1:
         training_args.distributed_state.distributed_type = DistributedType.DEEPSPEED
 
-    compute_dtype = (
-        torch.float16
-        if training_args.fp16
-        else (torch.bfloat16 if training_args.bf16 else torch.float32)
-    )
-
     local_rank = training_args.local_rank
 
     device_map = "auto"